Watt Fun (oz10889)

About this Plan

Watt-Fun. Small electric RC model. Wingspan 30in, wing area 210 sq in.

Quote: "A Small R/C Electric Powered Fun-Fly Model. Watt-Fun, by Bob Aberle.

We've all seen the traditional 0.40-sized, glow powered fun-fly models that have been around for years. In recent times I've seen many of these models flown with electric power, using 40- or 60-size geared motors and lots of battery cells. The idea behind my Watt-Fun design was to come up with a comparable fun fly design, but for a much smaller electric motor system.

My original goal was a 200-square-inch model, weighing about 10-12 ounces and possibly using as small a geared motor as an RS-280. I also wanted it to use more conventional-size radio system components to keep the cost down. Most important, I wanted a design that would only take a day or two to scratch-build from plans.

About the Design. The final Watt-Fun design ended up at 210 square inches, with a wingspan of 30 inches and an overall fuselage length of 24 inches. Depending on my choice of motor/battery systems, the total weight varied from 11.3 to 14.6 ounces. So even at the heaviest weight, the wing loading ended up a very respectable 10 oz/ sq ft.

To make the construction as easy as possible, I chose a common 5/16-inch-diameter carbon fiber control rod (Aerospace Composites, CST or Sullivan Products) for my fuselage. The rod attaches under the wing with enough projecting out the front for the motor and even more projecting out the rear to carry the vertical fin and stab. In the final analysis, you are build-ing essentially a wing structure. All of the radio equipment is built into the wing. The battery pack is just attached up on top with some hook and fastener tape.

Let's now get into some of the construction hints and then I will explain some of the motor system combinations that are possible with this design.

Construction: The wing was intentionally designed to have a rectangular shape so all the ribs would be of the same size. Although traditionally fun-fly models have very thick airfoil sections, the low power loading intended for this version could only tolerate a much thinner section. The choice was the ever-popular NACA 2412 airfoil, which was slightly modified at the trailing edge. Since it is a cambered section, the bottom also is rounded somewhat, making it difficult to build flat on a board. To solve that problem, I cut all the ribs out in stacks with a bunch of balsa blanks sand-wiched between two plywood tem-plates. By doing this, I was able to drill 1/8-inch-diameter holes in two places on each rib. This allowed me to use 1/8-inch-diameter wire rods as a build-ing jig. The photos show this more clearly. Those same wing jig holes later made a perfect passage for the internal radio antenna.

The key point to note in the wing structure is that the two center ribs are made from 1/16-inch plywood and project below the bottom of the wing. You arrange these two ribs at the centerline and space them exactly 5/16 inch apart. I used a scrap piece of 5/16-inch balsa for this purpose. These two plywood ribs have been carefully designed to help align the fuselage rod later on in the assembly. Once the basic wing structure is complete, you can proceed to make up the ailerons out of 1/8-inch balsa strips. I used Sonic-Tronics lite hinges, but, quite honestly, you could get away with just strips of covering material for hinges. The vertical fin, stab and elevators are likewise made from 1/8 balsa sticks and sheeting.

When you have all the components constructed, the assembly begins by attaching the 5/16-inch carbon rod (fuselage) to the lower half of the wing. The flat bottom of the two center ribs should be exactly parallel to the rod. The rod itself should be flush with the bottom of these ribs. When you have everything aligned, spot the rod in place with some CA cement. Then follow up with a coating of 5-minute epoxy. The vertical fin is installed next..."

Watt Fun, S&E Modeler, November 1999.

Direct submission to Outerzone.

Supplementary file notes

Article pages, thanks to RFJ.

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